Micro-electromechanical systems (MEMS) are used in a variety of applications such as optical display systems. Such MEMS devices have been developed using a variety of approaches. In one approach, a deformable deflective membrane is positioned over an electrode and is electrostatically attracted to the electrode. The gap between the two electrodes determines the output of the device. Accordingly, the output of the device is controlled by controlling the gap distance.
One approach for controlling the gap distance between electrodes is to apply a continuous control voltage to the electrodes, wherein the control voltage is increased to decrease the gap distance, and vice-versa. In such approaches the gap distance changes as charge accumulates on the electrodes, creating an electrostatic force therebetweeen. This electrostatic force is opposed by a mechanical restoring force provided by the deflection of flexures that supports one of the electrodes. This approach suffers from electrostatic instability that greatly reduces a usable operating range over which the gap distance can be effectively controlled. This is because the electrodes form a variable capacitor in which capacitance increases as the gap distance decreases.
When the gap distance is reduced to a certain threshold value, usually about two-thirds of an initial gap distance, the electrostatic force of attraction between the electrodes overcomes the mechanical restoring force causing the electrodes to “snap” together or to mechanical stops. This is because at a distance less than the minimum threshold value, the capacitance is increased to a point where excess charges are drawn on the electrodes resulting in increased electrostatic attraction. This phenomenon is known as “charge runaway.”
As introduced, the electrodes are sometimes snapped to mechanical stops. The size of the optical gap when the electrodes are in contact with the mechanical stops often corresponds to the black state size of the optical gap, such that when the electrodes are in this position, the device absorbs light incident thereon. This mechanical contact may result in the electrodes sticking together (or stiction). Further, this electrical contact may also result in spot welding. Accordingly, the contact may reduce the reliability and/or operating life of a device and consequently the display system that makes use of such a device.